The seven polypeptide chains which comprise the subunits of the keratin filaments of normal bovine epidermis have been isolated and characterized. The unfractionated polypeptides or mixtures of as few as two polymerize in vitro into filaments which have the same physical and chemical structure as the in situ keratin filaments. The stoichiometry of these recombinations and the alpha-helical type X-ray diffraction patterns displayed by the filaments indicate the presence of a three-chain unit structure for the epidermal filament. Studies on the mechanism of polymerization in vitro and characterization of the alpha-helical regions of the filaments will provide further information on the ultrastructure of the filaments. The keratin polypeptide subunits of abnormal and malignant human epidermis are different from those of normal epidermis and work is in progress to define these differences. Normal mouse epidermal cells grown in cell culture synthesize the keratin filaments as well as muscle actin-like protein. The appearance of these proteins in the cells grown in culture will be used as biochemical markers for studies of in vitro carcinogenesis. The actin-like protein is also present in a variety of human and mouse epidermal tumors and may be used as a marker for malignant tumors. BIBLIOGRAPHIC REFERENCES: Steinert, P.M.: The mechanism of assembly of bovine epidermal keratin filament in vitro. In Biochemistry of Cutaneous Epidermal Differentiation. Seiji, M. and Bernstein, I.A., editors. Univ. of Tokyo Press, Tokyo, 1977, pp. 444-466. Steinert, P.M., Idler, W.W. and Zimmerman, S.F.: The self-assembly of bovine epidermal keratin filament in vitro. J. Mol. Biol. 108: 547-567, 1977.